This invention relates to the preparation of copolycarbonates, and more particularly to a method for their preparation which includes solid state polymerization.
Solid state polymerization is disclosed, for example, in U.S. Pat. Nos. 4,948,871, 5,204,377 and 5,717,056, the disclosures of which are incorporated herein. It typically involves a first step of forming a prepolymer, typically by melt polymerization (i.e., transesterification) of a dihydroxyaromatic compound such as 2,2-bis(4-hydroxyphenyl)propane (bisphenol A) with a diaryl carbonate such as diphenyl carbonate; a second step of crystallizing the prepolymer; and a third step of building the molecular weight of the crystallized prepolymer by heating to a temperature between its glass transition temperature and its melting temperature. Use of this method is of increasing interest by reason of its effectiveness and environmental benefits.
The end uses for which the overall properties of polycarbonates make them particularly suitable potentially include many which require specific properties not possessed by aromatic polycarbonates as a genus, and in particular not possessed by bisphenol A polycarbonates. For example, polycarbonates are widely used for the fabrication of optical data-recording media, including optical disks as exemplified by compact audio disks and CD-ROM disks used in computers. By reason of the optical requirements of such disks, particularly those having read-write capability, it is essential or at least strongly preferred that they have low birefringence. Birefringence is the difference between indices of refraction for light polarized in perpendicular directions. Birefringence leads to phase retardation between different polarization components of the laser beam which reads the optical disk, thereby reducing readability. Polycarbonates prepared from bisphenol A have relatively high birefringence, which is typically lowered for optical purposes by incorporation of such monomers as 6,6'-dihdyroxy-3,3,3',3'-tetramethyl-1,1'-spiro(bis)indane, hereinafter designated "SBI", or 1,1,3-trimethyl-3-(4-hydroxyphenyl)-5-hydroxyindane ("CD-1").
It is also frequently desirable to improve the processability of polycarbonates by the incorporation, for example, of "soft blocks" derived from aliphatic compounds such as alkanedioic acids or polyoxyalkylene glycols. This is particularly important in optical disk fabrication, since processability is generally degraded by the presence of such monomers as SBI and CD-1 as a result of such factors as increase in glass transition temperature.
Typical interfacial methods of polycarbonate preparation, by the reaction of one or more dihydroxyaromatic compounds under alkaline conditions with phosgene in a mixed aqueous-organic system, are frequently not adaptable to preparation of "soft block" polycarbonates since the alkaline environment can degrade the soft block monomer. Melt (transesterification) methods, by the reaction of the dihydroxyaromatic compounds with diaryl carbonate, are likewise of limited use because the high temperatures (on the order of 300.degree. C.) required for building a high molecular weight polymer can cause thermal decomposition of the soft blocks.
The use of solid state polymerization (hereinafter sometimes "SSP") conditions, which are usually milder and require lower polymerization temperatures than melt polymerization, could be of use for the preparation of copolycarbonates containing soft blocks and/or birefringence-decreasing units. However, many monomers, especially the soft block ones, may undergo degradation even under SSP conditions even though the structural units derived therefrom may be stable under the same conditions. An alternative method of incorporating the copolymeric units in a polymer ultimately produced by SSP is therefore necessary.